This invention relates to an improved continuous process for the carbonylation of ethylene, in particular, a method providing an improved turnover number (TON) for the catalyst system employed in the carbonylation.
The carbonylation of ethylenically unsaturated compounds using carbon monoxide in the presence of an alcohol or water and a catalyst system comprising a group 6, 8, 9 or 10 metal, for example, palladium, and a phosphine ligand, for example an alkyl phosphine, cycloalkyl phosphine, aryl phosphine, pyridyl phosphine or bidentate phosphine, has been described in numerous European patents and patent applications, for example EP-A-0055875, EP-A-04489472, EP-A-0106379, EP-A-0235864, EP-A-0274795, EP-A-0499329, EP-A-0386833, EP-A-0441447, EP-A-0489472, EP-A-0282142, EP-A-0227160, EP-A-0495547 and EP-A-0495548. In particular, EP-A-0227160, EP-A-0495547 and EP-A-0495548 disclose that bidentate phosphine ligands provide catalyst systems which enable high reaction rates to be achieved. C3 alkyl bridges between the phosphorus atoms are exemplified in EP0495548 together with tertiary butyl substituents on the phosphorus.
WO96/19434 subsequently disclosed that a particular group of bidentate phosphine compounds having an aryl bridge could provide remarkably stable catalysts which require little or no replenishment; that use of such bidentate catalysts leads to reaction rates which are significantly higher than those previously disclosed; and that little or no impurities are produced at high conversions.
WO 01/68583 discloses rates for the same process as WO 96/19434 when used for higher alkenes and when in the presence of an externally added aprotic solvent.
WO 98/42717 discloses a modification to the bidentate phosphines used in EP0495548 wherein one or both phosphorus atoms are incorporated into an optionally substituted 2-phospha-tricyclo[3.3.1.1{3,7}]decyl group or a derivative thereof in which one or more of the carbon atoms are replaced by heteroatoms (“2-PA” group). The examples include a number of alkoxycarbonylations of ethene, propene and some higher terminal and internal olefins.
WO 03/070370 extends the teaching of WO 98/42717 to bidentate phosphines having 1, 2 substituted aryl bridges of the type disclosed in WO96/19434. The suitable olefin substrates disclosed include several types having various substituents.
WO 04/103948 describes both the above types of ligand bridges as useful for 1,3-butadiene carbonylation and WO 05/082830 describes a selection of WO 04/103948 where the tertiary carbon substituents are different from each other on the respective phosphorus atoms.
EP0970038A1 discloses a liquid phase reaction using carbon monoxide and a catalyst system for the carbonylation of ethylene. The gas phase ratio of ethylene:CO is held above 1:1. The preferred range is 7:1 to 15:1 which is said to improve the TON of the catalyst. At the same time the ethylene:CO incoming gas feed is taught at 1:1 to replenish the ethylene and carbon monoxide consumed in the reaction. However, it has now been surprisingly found that in a continuous process a higher gas phase ratio and a higher feed rate gives an even further improvement in the TON.